Binocular vision is such a natural experience that we take it for granted. The stream of images from each eye is fused within the brain to create a single view of the world. Within weeks of birth, infants learn to align and move their eyes together, so that targets are projected accurately onto each retina. Once the eyes become aligned, binocular depth perception begins to emerge. Unfortunately, in 2% of children, ocular alignment fails, giving rise to strabismus. When the eyes become misaligned in adults, the result is double vision or diplopia. When strabismus develops in children, diplopia also occurs, but only transiently. Within days, children suppress one image. This sensory adaptation is helpful, because it eliminates diplopia. However, it is also harmful, because it deprives the visual system of the feedback required to maintain ocular alignment. Instead of adjusting the eye muscles to re-align the globes, children tolerate strabismus. The deviated eye often develops amblyopia, or lazy eye, because of chronic suppression. A major goal of this research project is to learn how suppression occurs in the brain. Recently, we have performed dichoptic visual field mapping in human patients to define the pattern of visual suppression that occurs in exotropia, or outwards eye deviation. We have also raised monkeys with exotropia, by weakening the medial rectus eye muscles after birth. These animals display patterns of visual suppression nearly identical to those in humans. To have a macaque model of strabismus is valuable, because it is possible to record the electrical discharges of single neurons to define the basic neural mechanism of suppression. In Specific Aim #1, we will record from the primary visual cortex of macaques with strabismus, in whom suppression scotomas have been previously mapped, to compare the responses of single units during monocular versus binocular stimulation. The hypothesis is that cells with receptive fields located within an eye's suppression scotoma may respond to monocular stimulation, but will be inhibited when the other, dominant eye is also stimulated. In Specific Aim #2, we will examine how human subjects with strabismus decide which eye to use to look at targets in their visual environment. After mapping of suppression scotomas, subjects will be presented with peripheral targets visible to only the right eye, the left eye, or potentialy either eye. For each location in the visual field, we will determine if the eye that acquired the target was the eye that perceived the target. This experiment will reveal whether people with strabismus sometimes use one eye to acquire target location, and the other eye to saccade to it. In Specific Aim #3, we will investigate the topography and properties of cells in the superior colliculus, a brainstem center that controls eye movements, to probe the neural mechanisms governing eye selection for saccades in primates with strabismus. Knowledge gained from this work may provide a new basis for developing strategies to prevent and treat strabismus.